1. Field of the Invention
The present invention relates to illumination and, more particularly, to the use of coherent light sources as illuminators.
2. Description of the Related Art
The prior art of illumination system design includes many examples of illumination designs. There are broadly two different design approaches used when incoherent sources are used. The first approach is to use a single source with one or more optical elements to spread the available radiant power or energy over one or more target zones. In this first approach, significant trade-offs are made between various performance variables. Typically source size is increased to achieve greater radiance values at the expense of collimation. Lossy filters have to be used to achieve any desired spatial gradients in wavelength or polarization. Optics of various description have been added to shape the distribution of incident power, always with incumbent losses. Examples of implementations that use this design approach include flashlights, beacons, projectors, and desk lamps.
The second design approach using incoherent sources is to use more than a single source, often in combination with respective optics. Typically this approach is found only in large scale illumination systems such as an array of street lamps, banks of lamps used for drying painted surfaces, or the headlights on an automotive vehicle. Examples also exist of the use of multiple sources with a single optic such as an array of LEDs along the length of a cylindrical lens to form a strip of illumination as in a fax machine.
Unlike incoherent sources, lasers provide polarized monochromatic light with a spatial coherence that permits their output power to be efficiently directed with low beam divergence. However, a significant tradeoff is required if the illumination must provide uniform radiant incident power (irradiance) across a target zone. Such uniformity is typically achieved by wasting power outside the target zone, attenuating the brightest portions of the beam, and/or using holographic optical elements.
Unlike the many applications of multiple incoherent illumination sources, most applications of lasers have been designed around use of a single laser. One use of multiple laser beams converging into a common target zone simultaneously has been made at Lawrence Livermore National Laboratory, Livermore Calif., to compress a plutonium pellet in the creation of nuclear fusion. Multiple lasers are also popular in entertainment, but the objective there is to create a non-uniform and varying illumination field.
Thus, it can be seen that competing design constraints impose limits upon current-technology illumination devices and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for an illumination device that will significantly improve energy and/or power efficiency of current-technology illuminators while enabling efficient control of performance variables throughout a prescribed target zone.